app_config.xml: max_concurrent ignored?

Using the app_config.xml shown below, my Linux host with two AMD GPUs and a 4-core CPU had been running, as expected, 6 tasks with the Gamma-ray pulsar binary search #1 on GPUs v1.18 application and 2 tasks with the Continuous Gravitational Wave search O2 All-Sky v1.01application (a.k.a. GW CPU). (Current Project Preferences are set to not run beta test applications, so no O2AS20-500 v1.07 GPU work):

<app_config>
  <app>
    <name>hsgamma_FGRPB1G</name>
    <fraction_done_exact/>
    <gpu_versions>
      <gpu_usage>0.333</gpu_usage>
      <cpu_usage>0.4</cpu_usage>
    </gpu_versions>
  </app>
  <app>
    <name>einstein_O2AS20-500</name>
    <fraction_done_exact/>
    <gpu_versions>
      <gpu_usage>0.33</gpu_usage>
      <cpu_usage>0.4</cpu_usage>
    </gpu_versions>
  </app>
  <app_version>
    <app_name>einstein_O2AS20-500</app_name>
    <plan_class>LIBC215</plan_class>
    <avg_ncpus>0.7</avg_ncpus>
    <max_concurrent>2</max_concurrent>
  </app_version>
<project_max_concurrent>8</project_max_concurrent>
</app_config>

When the following mysterious, but transient, error popped up in my Event Log: "platform 'x86_64-pc-linux-gnu' not found", the host stopped getting downloads of the GPU tasks. (The error initiated a 24 hr backoff, but when I prompted a project update a few hours into the backoff, it began downloading GPU tasks again and tasks ran as expected.) But before things returned to normal, when the FGRPB1G GPU queue ran out, BOINC began running five concurrent GW CPU tasks. With how app_config is set up, I expected it to just continue running two concurrent GW CPU tasks.

So why did the <max_concurrent>2</max_concurrent> setting for O2AS20-500 CPU work not hold in the absence of FGRPB1G GPU work? And why five concurrent CPU tasks and not eight? Was it because that's all that could fit onto 4 cores with <avg_ncpus>0.7</avg_ncpus>?